Dust bunnies are probably the best metaphor when trying to understand how planets form. (Although stars form much in the same way, its rare and difficult to see.)
It's simply a property of matter to coalesce or aggregate, specially when in a vortex with a proper mix of chaos. Astrophysicists have recently captured images of a "dust bunny" in the process of becoming a planet. The star is AB Aurigae, and here is the link for more details about the evidence. The team who captured this image used the technique caled coronagraphy, which blocks out the brightest light emitted by the star to permit dimmer objects nearby to be seen. Another technique is polarimetry, which filters out the starlight with incredible precision.